Impact of the Specific Surface Area on the Memory Effect in Li‐Ion Batteries: The Case of Anatase TiO₂

Abstract

solid electrolyte interphase. On the other hand, increasing the surface area shortens the length of Li-ion diffusion pathways and decreases the Ohmic drop within the material. The mechanism and properties of Li-ion (de-)intercalation for TiO 2 was shown to be dependent on the particle size for some materials. [ 7,8 ] It was proposed that both Li-rich and Li-poor phases cannot co-exist within one TiO 2 particle for particle sizes below a certain value, [ 7 ] which could infl uence the potential memory effect. These facts suggest the question: If the memory effect could be one of those particle size dependent properties?Here, we reveal the presence of a memory effect in anatase TiO 2 , showing that other active materials, beside LiFePO 4 , may suffer from a memory effect in LIBs. We also demonstrate that the memory effect in anatase TiO 2 is dependent on the specifi c surface area. Namely, increasing the surface area of the TiO 2 material leads to a decrease in the memory effect, which is practically disappearing for the particles with a very high specifi c surface area (300 m 2 g −1 ).We prepared seven samples of anatase TiO 2 with different specifi c surface areas (referred to as TiO 2 _ X , where X is the specifi c surface area of 10, 60, 95, 130, 180, 240, and 300 m 2 g −1 ). The values of the specifi c surface area were obtained by the Brunauer-Emmett-Teller method and the anatase phase was confi rmed for all samples by X-ray diffraction (Supporting Information Figure S1). For the evaluation of the memory effect in these samples, we followed the electrochemical procedure as suggested by Sasaki et al., [ 5 ] which consists of applying three subsequent cycles: a normal (de-)intercalation cycle, a memory writing cycle and a memory releasing cycle ( Figure 1 a). During the memory writing cycle, the oxidation process was interrupted at a certain SOC. In the next memory releasing step the sample was cycled within the normal potential range, i.e., 3 V to 1 V in our case, and the memory effect was revealed if occurred. For all examined materials, the memory effect was observed in the memory releasing cycle. It appeared as a bump in the plateau on Li-ion de-intercalation occurring at a similar SOC at which the memory writing cycle was stopped (Figure 1 b,c) similarly as in case of LiFePO 4 . [ 5 ] Here, we defi ne the magnitude of the memory effect as a difference (in mV) between the potential of the maximum of the memory effect bump and the potential at that SOC during the normal cycle (Figure 1 a). Figure 1 presents the data for TiO 2 _95 cycled at C/3 rate, but similar behavior was observed for each examined sample (Supporting Information Figure S2a-f).It is known that the two phases do not coexist within one particle during the phase transformation for some of the materials with very small particles. [ 7 ] Instead, they "switch" spontaneously from one phase to another when the critical Li-ions content is reached and this transformation takes place at once within the entire particle. The phase transformation results...

Explaining key properties of lithiation in TiO2 -anatase Li-ion battery electrodes using phase-field modeling

“…In the memory cycle, the regions with poor diffusion will increase the charge-transfer overpotential in comparison to completely delithiated particles, explaining the observed memory effect. The phase-field simulations show that the charge-transfer overpotential strongly depends on the applied C rate and the surface to volume ratio, explaining the experimental observations [76] of a larger memory effect at high C rates and with smaller surface to volume ratios.…”

“…Experimentally, similar observations are reported upon increasing the surface area [74][75][76]. However, higher surface areas are usually achieved by reducing the particle size [74,75], which has a similar impact on the voltage profiles, as shown in Fig.…”

“…The strongly concentration-dependent Li diffusion provides an explanation of the memory effect observed for anatase electrodes [76], which presents itself through a decrease in the voltage in the charge cycle (lithiation) following upon an incomplete charge/discharge cycle. The memory effect is caused by particles that are not completely delithiated before they are lithiating again, and thus more regions with poor diffusivity will remain in comparison to a complete charge/discharge cycle.…”

“…This leads to larger capacities, as has been demonstrated by various experimental studies on high surface area anatase particles [74][75][76]. To capture the surface area effect simulations were performed on a spherical and a cylindrical particle (infinitely long, i.e., neglecting the top and bottom surface of the cylinder).…”

Carbon-Coated Anatase TiO2Nanotubes for Li- and Na-Ion Anodes

“…85 Other studies on anatase TiO 2 nanoparticles, investigating various samples with different crystallite sizes, confirm these findings, which may be explained considering the following points. 29,30,55,82,87 First, the extension of the sloped low potential region might partially originate from the presence of amorphous TiO 2 , for which generally no voltage plateau is observed. 84,88 Second, the formation of the poorly ionically conductive Li 1 TiO 2 phase at the particles surface is much more pronounced for smaller particles, which are intrinsically characterized by a higher surface area, thus hindering further lithium insertion.…”